1. Field of the Invention
The present invention relates to a molding roll for continuously manufacturing substrates for high-density information storage media such as optical discs and optical cards which can record and reproduce information by changing optical characteristics such as reflectance with light of a laser beam or the like.
2. Related Background Art
Heretofore, an optical recording medium has been formed by providing a recording layer on a substrate transparent to light such as a semiconductor laser by which recording and playback of information are carried out, and then laminating a protective layer on the recording layer. On the surface of this substrate for the optical recording medium, there are engraved fine preformats of micron order or submicron order such as grooves for tracking and address information pits. As conventional methods for forming these preformats, for example, an injection molding method and a compression molding method are known. However, these methods are unsatisfactory regarding mass productivity and cost.
In order to solve this problem, the present applicant has suggested, in EP-A-0 369 780 and EP-A-0 369 781, methods for preparing substrate sheets for optical recording media which comprises the steps of melting and extruding a resin to mold resin sheets, and then pressing the molded resin sheets between a molding roll and an oppositely disposed roll to transfer preformats thereto.
According to the above-mentioned methods, substrate sheets for optical recording media can be continuously formed which are excellent in transferability of the performats. In addition, double refraction which will be the cause of the decline in a C/N value is controlled, even when a resin such as polycarbonate which easily causes double refraction is used.
However, as a result of the investigations on the above-mentioned methods, it has been found that values of the double refraction and the transfer accuracy of the substrate sheet change owing to the slight alteration of conditions at the time of the molding, for example, the heating temperature of an extruder, heating temperatures of a molding roll and mirror rolls and room temperature at the time of the molding. The above-mentioned fluctuation of the double refraction and the transfer accuracy is not preferable even if it is in an allowable range, because it interferes with stabilizing the performance of the optical recording media.
The present inventors have investigated the above-mentioned problem in detail, and as a result, have found the following. For example, as described in the above-mentioned EP-A-0 369 780, in the case that the substrate sheet for the optical recording medium is molded by the use of three rolls as shown in FIG. 8 and when the high-quality substrate sheet is desired, a molded resin sheet 82 extruded from a T-die 81 is maintained in a nearly melting state in being pressed between a first roll 83 and a molding roll 84 so as not to cause the double refraction (so as not to fix stress to be applied to the resin) and in order to improve the transferability. In addition, when the molded resin sheet is smoothly transferred from the molding roll 84 to a third roll 85, it is necessary that the molded resin sheet is cooled not to adhere to the molding roll. However, these requirements have been heretofore met by adjusting the molding conditions. Thus, when the high-quality substrate sheet for the optical recording medium is molded, particularly when the temperature of the resin and the molding roll shifts to a lower temperature side, the margin of the molding conditions has been limited.